"one in four knows a trans person personally"
This is extremely misleading, and a case of ridiculous scaling. It's like surveying members of the National Gerbil Society and pronouncing that everyone in Britain buys dry seed mix.
I am in a position where I know two transwomen, both with GRCs. I also know that they are the only known transwomen with GRCs within an area of roughly a 70,000 population, and they are probably the only individuals with GRCs within a 210,000 population area. I also know who is not aware of these two people and, considering they are near enough recluses, it's practically everybody.
However, if you use a wider parameter for the definition of "trans", and include children, teenagers and young adults, who may be treating the label as a form of subculture, then the number expands considerably. But it is still not one in four by any means: that would suggest 25% of the population knows a transperson.
Going by a rough estimate of a 65 million population, this would mean that over 16 million people in Britain know a transperson.
As a useful comparison, there's only around 9 million people in London, so you are talking an extra 7 million on top of that (and Scotland is only about 5.5 million).
It also means that if you got every person in Britain who knew a transperson, according to this frankly silly research conclusion, to stand at half mile intervals, the line would stretch from the UK to East Timor.
That no one pointed this out to anyone at the Guardian says quite a lot about the rigor of their editors and subeditors, in my view.